Soil Compaction.
- Definition.
The increase in
density of a soil material usually by mechanical mean, although hand tampers
are also used in confined space.
The soil is packed
together tightly to expel the air voids, producing a heavier unit weight,
higher strength, and stability.
Compaction could also
occur in nature as a result of overburden compressing lower soils, expelling
water over an extended time period.
- The method of measuring soil compaction in the laboratory.
To express the degree
of soil compaction and measure his degree with standard specifications for this
degree of compaction, the standard or modified Proctor method may be used. We
already explain this test method in previous article (modified Proctor test).
- Factors that affect the degree of compaction.
The value of the
maximum dry density of the soil as well as the corresponding fundamental
moisture ratio depends on several factors including:
- The type of soil compacted.
Coarse-grained soils
have a high maximum dry density accompanied by a smaller original moisture
content in comparison with fine particles whose maximum dry density is
relatively small and with higher relative moisture.
- The amount of energy or work expended in the compaction.
As the increase in the
compaction energy on the soil, increase the dry density value of it and thus
its maximum dry density. Meaning that the maximum dry density of the soil by
the modified Proctor method is greater than its counterpart by the standard
Proctor method, and this is followed by a decrease in the original moisture
content.
That is, there is an
inverse relationship that, with the increase of the maximum dry soil density,
its fundamental moisture percentage decrease for the same type of compacted
soil.
- The water content of soil.
The water content of
soil which is that the higher the water content of the soil, the higher the dry
density to a certain extent (reaching the maximum dry density) then the dry
density decrease with the increase of the water content.
- Compaction soil at site.
- Earth equipment.
Compaction are done in
the site using different equipment, each suitable for a specific type or types
of soil. Equipment including:
- Walk-behind plate
compactor.
This is steel plate
equipped with a motorized vibrator that can be handled by personnel walking
behind it and guiding its movements.
The vibration permits
the plate to be readily moved to compact soil in a small area not accessible to
large compaction equipment. Because of the low contact pressure of the plate
compactors, the process should be applied to thin soil lifts.
- Vibratory Pad-Foot
Roller.
Used to compact clay
or granular soils. It is most effective for granular or sandy soils. Pads can
be round or square. The taper of the pads reduces soil fluffing during pad
up-lift. The drum width is 84 inches and rate of production of compacted road
base or soil is about 500 CU. YDS/H.
Vibration frequency
and roller weight can be adjusted to provide optimum results. Vibration
frequency is standard at 1914 VPM up to 1800 VPM for large models. Vibration
amplitude can be either 0.070 inches or 0.035 inches producing centrifugal
force of 63.300 LBS or 31.600 LBS. for heavy clay, the tapered-foot 4-wheel
compactor or the older sheet foot rollers are most effective.
- Vibration drum
roller.
Vibration drum roller
is similar to the pad-foot roller but uses a smooth roller drum. It is often
used to smooth and finish a sub-grade, the surface of pavement base courses, or
top off building foundation pads.
- Four-wheel soil
compactor.
It is primarily used
for compaction heavy clay soil. The tamping wheel tip configuration has a
“chevron” shape to increase maximum ground pressure. Similar to the tapered-pad
machines, compaction is from the bottom of the lift, and the tips walk out when
compaction is achieved. The blade also permits spreading fill. Depending on
slope, blade load, and gear selection, the compactor can operate at 4 to 12
MPH. operating weight is about 46.000 LBS.
- Pneumatic roller.
The total weight of
this machine ranges from 19000 LBS. empty to 55000 LBS with maximum ballast
load. Ballast consists of water, steel bolt-on weights, steel weights plus
water, and steel plus wet sand. The pneumatic tires produce an eccentric or
wobble motion that serves to knead the soil. Tier air pressure can range from
35 to 110 PSI. the rubber tiers can have the contact pressure adjusted by air
pressure and change pf ballast. Ground contact pressure can be varied from 71
to 83 PSI average. Maximum contact pressures can range from 130 to 140 PSI
since the contact pressure is not uniform. Rolling speed is about 5 MPH and
compaction width is 90 inches. The best use of this machine is to compact sandy
clay and crushed stone pavement base. Heavy clay is best compacted using a
tapered-foot compactor.
- Determine the degree of relative compaction of the soil at the site.
In order to determine the
degree of relative compaction of the soil in the site after its compaction, the
standard cone test (sand cone) is performed to determine the dry density of the
soil at the site after determining its total density and moisture content for
any compacted layer at the site, and at the same time a sample is taken from the
same soil compacted in the site and on which a standard compaction test is performed
in the laboratory using the modified Proctor method where the maximum dry soil
density is determined in the laboratory. The degree of relative soil compaction
in the site is define as the ratio between the value of the dry soil density at
the site (γsite) and the maximum
density dry soil in the lab (γdmax), so we get a value less than one that expresses the degree of relative
compaction.
For example,
Dr = γd site / γdmax < 1.0
It should be noted
that the greater the degree of compaction of soil in the site, that is, the
closer to the correct 1.0, Whenever indicates the high degree of compaction at the site.
In addition, that
using this method is possible to developed specification for compaction of the
soil at the site. requested a process from the contractor to compact the soil
to a dry density, given the 95% of its maximum dry density, given by the
standard Proctor method. When the contractor completes the implementation of a
layer of compacted soil (about 20 cm to a maximum of 30 cm), an experiment is
conducted to determine the dry density at the site as per previously mentioned
to ensure that the soil reach the required degree of compaction. If the contractor
fails to achieve this required compaction (95%), he must repeat the compaction
layer again and repeat the experiment after that.